Various types of hydrocarbon conversion reaction systems have found widespread utilization throughout the petroleum and petrochemical industries for effecting the conversion of hydrocarbons to a multitudinous number of products. The reactions employed in such systems are either exothermic or endothermic, and usually result in either the net production of hydrogen or the net consumption of hydrogen. These hydrocarbon conversion reactions include those which predominate in catalytic reforming, ethylbenzene dehydrogenation to styrene, propane and butane dehydrogenation, etc.
Petroleum refineries and petrochemical complexes are customarily comprised of numerous reaction systems. Some systems will be net consumers of hydrogen while other systems within the refinery or petrochemical complex may result in the net production of hydrogen. Net hydrogen refers to either the hydrogen which is available from a reaction for use elsewhere or the the hydrogen which must be added to a reaction from a source outside the reaction system. Because hydrogen is a relatively expensive substance, it has become the practice within the art of hydrocarbon conversion to supply hydrogen from reaction systems in which there is net production of hydrogen to reaction systems which are net consumers of hydrogen. Occasionally the hydrogen being passed to the net hydrogen-consuming reaction systems must be of high purity due to the reaction conditions and/or the catalyst employed in the systems. Such a situation may require treatment of the hydrogen from the net hydrogen-producing reaction systems to remove hydrogen sulfide, light hydrocarbons, etc., from the net hydrogen stream.
In some cases, the hydrogen balance for the entire petroleum refinery or petrochemical complex is such that there is excess hydrogen, i.e., the net hydrogen-producing reaction systems produce more hydrogen than is necessary for the net hydrogen-consuming reaction systems. When such is the case, the excess hydrogen may be sent to the petroleum refinery or petrochemical complex fuel system. However, because the excess hydrogen often has admixed therewith valuable components, such as C.sub.3 + hydrocarbons, it is frequently desirable to treat the excess hydrogen to recover these components prior to its passage to fuel.
Typical of the net hydrogen-producing hydrocarbon reaction systems are catalytic reforming, catalytic dehydrogenation of alkyl-aromatics, dehydrocyclodimerization (primarily aromatization of propane), and catalytic dehydrogenation of paraffins. Commonly employed net hydrogen-consuming reaction systems are hydrotreating, hydrocracking and catalytic hydrogenation. Of the above mentioned net hydrogen-producing and consuming hydrocarbon reaction systems, catalytic reforming ranks as one of the most widely employed. By virtue of its wide application and its utilization as a primary source of hydrogen for the net hydrogen-consuming reaction systems, catalytic reforming has become well known in the art of hydrocarbon conversion reaction systems. Accordingly the following discussion of the invention will be in reference to its application to a catalytic reforming reaction system. However, the following discussion should not be considered as unduly limiting the broad scope of the invention, which has wide application in many hydrocarbon conversion reaction systems. For example, another application is to a catalytic process referred to as dehydrocyclodimerization, wherein two or more molecules of a light aliphatic hydrocarbon, such as propane, are joined together to form a product aromatic hydrocarbon. Those having ordinary skill in the art will well recognize the broad application of the present invention and the following will enable them to apply the invention in all its multitudinous embodiments.
It is well known that high quality petroleum products in the gasoline boiling range including, for example, aromatic hydrocarbons such as benzene, toluene, and the xylenes, are produced by a catalytic reforming process where a naphtha fraction is passed to a reaction zone and contacted with a platinum-containing catalyst in the presence of hydrogen. Generally, the catalytic reforming reaction zone effluent, comprising gasoline boiling range hydrocarbons, light hydrocarbons, and hydrogen, is passed to a vapor-liquid equilibrium separation zone and is therein separated into a hydrogen-containing vapor phase and an unstablized hydrocarbon liquid phase. A portion of the hydrogen-containing vapor phase may be recycled to the reaction zone. The remaining hydrogen-containing vapor phase is available for use either by the net hydrogen-consuming processes or as fuel for the petroluem refinery or petrochemical complex fuel system.
Because the dehydrogenation of naphthenic hydrocarbons is one of the predominant reactions of a reforming process, substantial amounts of hydrogen are generated within a catalytic reforming reaction zone. Accordingly a net excess of hydrogen is available for use as fuel or for use in a net hydrogen-consuming process, such as the hydrotreating of sulfur-containing petroleum feedstocks. However, catalytic reforming also involves a hydrocracking function, among the products of which are relatively low molecular weight hydrocarbons, including methane, ethane, propane, butanes and pentanes. Substantial amounts of these appear in the hydrogen-containing vapor phase which is separated from the reforming reaction zone effluent. These normally gaseous hydrocarbons have the effect of lowering the hydrogen purity of the hydrogen-containing vapor phase to the extent that purification is often required before the hydrogen is suitable for other uses. Moreover, if the net excess hydrogen is intended for use as fuel in the refinery or petrochemical complex fuel system, it is frequently desirable to maximize the recovery of C.sub.3 + hydrocarbons, which are valuable as products or feedstock for other processes. It is therefore advantageous to devise a method of purifying the hydrogen-containing vapor phase to produce a hydrogen-rich gas stream and to recover valuable components such as C.sub.3 + hydrocarbons.
Separation of hydrogen from the hydrocarbon conversion products of a hydrogen-producing hydrocarbon conversion process is generally effected by cooling the reactor effluent and separating, by means of a vapor-liquid equilibrium separation vessel, a hydrogen-rich vapor phase and a liquid hydrocarbon phase. The hydrogen-containing vapor phase is often subsequently recontacted with at least a portion of the liquid hydrocarbon phase, whereby residual hydrocarbons are absorbed from the vapor phase into the liquid hydrocarbon phase. This recontacting process may be repeated one or more times, generally at increasingly higher pressures, to enhance the purity of the hydrogen-containing vapor phase and also enhance the recovery of hydrocarbon conversion products.
The liquid hydrocarbon phase is subsequently treated in a fractionation zone which is comprised of one or more fractionation columns and equipment which is auxiliary thereto, such as heat exchangers, pumps, and separators. The first fractionation column in the fractionation zone is often a stabilizer or debutanizer. The bottoms product from a debutanizer comprises C.sub.5 + hydrocarbons. The term "stabilizer" is used when significant amounts of butane are left in the heavy hydrocarbon product stream. The overhead component from the debutanizer or stabilizer column is cooled and passed to a vapor-liquid separator to provide two overhead products, overhead vapor and overhead liquid. The overhead vapor is comprised primarily of hydrogen and C.sub.4 - hydrocarbons and is normally used as fuel. Net overhead liquid consists primarily of C.sub.2, C.sub.3, and C.sub.4 hydrocarbons and it is often processed further to obtain a butane fraction and a propane fraction.
The overhead liquid may be treated in a fractionation column commonly known as a deethanizer, where C.sub.2 - hydrocarbons are removed as an overhead vapor stream for use as fuel. The deethanizer bottoms stream is usually fed to another fractionation column for separation into propane and butane.